OK, actually editing in 320x240 is the only way to make stuff full-screen without losing res so you're lucky. Take the clip that you want to make full screen into dvd2avi and create the d2v, make an avisynth script like
mpeg2source("yadda/yeah.d2v")
NOW, you have to find out first if you're using anamorphic footage or letterboxed. So after you add the mpeg2source line open the .avs file in virtualdub. If there are big black borders at the top and bottom its letterboxed. If the image is fullscreen but everything lookes stretched and tall then its anamorphic.
Either way add the decomb script but dont include decimate. Like this
Telecide()
For letterboxed, first resize to 640x480 with something like
BicubicResize(640,480,0.0,0.5)
then manually adjust the crop command in avisynth until you cut off exactly the top and bottom black bars. It might be something like
Crop(0,60,-0,-60)
the syntax for that is
crop(pixels from left, pixels from top, -pixels from right, -pixels from bottom).
Now a good idea is to open the file in virtualdub and check the exact resolution with File>File information. Then input the width and height into klinky's lovely
MisatoAspect and 1 for the divisible by box. One of the numbers should have a height of 240. Use bicubicresize again
BicubicResize(whatever misatoaspect told you,240,0.0,0.5)
now crop to 320x240, by taking what misatoaspect had (say 448) and subtracting it by 320. In this case 128. Divide that by 2, i get 64. So my crop would be
crop(64,0,320,240)
Take that into virtualdub and export and it should be perfect.
Tell me if your footage is anamorphic cuz I dont feel like writing that out right now if it isnt ~_~